Transistors are currently used in a wide variety of applications such as signal modulation, signal regulation and amplification, memory circuits, and signal switching. Two common types of transistors are bipolar junction transistors (BJT) and field-effect transistors (FET). A BJT typically has three terminals labeled emitter, collector, and base. An FET transistor typically has three main terminals commonly referred to as gate, drain, and source. Many FETs have a fourth terminal commonly referred to as the body.